This invention relates to a method of filtering a gas containing fine particles by means of a filter, and to a filtration apparatus for practicing the method.
Various methods of filtration are known in the art for completing removing fine particles from a gas to fully recover the particles and clean the gas. Such methods rely upon an electrostatic Cottrell precipitator or an absorption tower which uses a liquid-based system, but the simplest and most reliable method in wide use is to perform filtration by means of a filtration apparatus having a filter consisting of a filter cloth. A typical example of such a filtration apparatus has a plurality of the filters, each in the form of a cylinder, so arranged as to depend vertically from the ceiling plate of a collecting vessel. The contaminated gas is caused to pass through each cylindrical filter from the outer to the inner side thereof, whereby the particulate matter contained in the gas is removed. The cleaned gas then exits from the apparatus through passages penetrating the ceiling plate above respective ones of the cylindrical filters.
The filter cloth used in the aforementioned filters may be broadly classified into two types. The first is a thick unwoven fabric having an extremely coarse texture and a thickness of from 15 to 30 mm, and the second also is an unwoven fabric having a fine surface and a thickness of from 1 to 2 mm.
With the first filter cloth, particulate matter such as dust contained in the gas (which we will hereinafter assume to be air for the purpose of description) is caused to lodge within the filter texture thereby to be absorbed within the walls of the filter. The purpose of this filter is to incompletely collect the dust contained in the air stream only at a low concentration. Thus the particulate matter is removed imperfectly and cannot be reutilized.
The second filter cloth, on the other hand, does not ordinarily have a pore diameter as small as the particle diameter of the particles that are desired to be collected. Specifically, the mean pore diameter in many cases ranges from 5 to 20 microns. Filtration of this kind belongs to the category of so-called cake filtration. Specifically, using a filter cloth of a pore diameter greater than the particle diameter of the particulate matter, a very minute proportion of the particulate matter passes completely through to the secondary side (inner side) of the filter cloth at the instant filtration begins, but immediately thereafter the particulate matter forms a layer, known as a filter cake, on the primary or outer surface of the filter cloth. The filter cake itself then begins acting as a filter to thenceforth enable complete collection of the particles.
The cake filtration method using the second filter cloth is beset by a number of problems when filtration is conducted at a high rate. With a high rate of filtration, the concommitant wind pressure causes the particles in the filter cake to penetrate through the pores in the filter cloth and emerge from the secondary side. This is referred to as so-called filtering break-through, making it impossible to achieve full collection of the particles. The high wind pressure also drives the particles deeply into the filter cloth texture, clogging the pores so that recovery by means of shaking or backwashing cannot be achieved with satisfactory results.
Owing to the aforementioned problems, the flow velocity through the second filter cloth that is adopted for particle collection is ordinarily from 1 to 2 m/min (1.67 to 3.33 cm/sec). This is much lower than the flow velocity of 1 to 2 m/sec that can be realized with the first filter cloth. Accordingly, for a constant air flow rate, it is required that the second filter cloth have a very large area to assure proper operation. However, for certain particulate matter which is easy to deal with, depending upon such factors as the particle diameter and adherability, a flow velocity of up to 6 m/min (10 cm/sec) can be achieved.
There are other reasons for requiring the very low filtration flow speed mentioned above. For instance, a high flow velocity and small filter area causes an abrupt rise in the pressure differential across the wall of the filter cloth owing to its fine texture. Also, a high flow velocity makes it difficult to collect the particles fully and to recover dust following removal.
When filtration is carried out with a filtration apparatus that relies upon the filter cloth of the second type, the particles extracted from the air form a filter cake by attaching to and accumulating on the upstream side (namely the primary or outer side mentioned above, the opposite or downstream side being the secondary or inner side) of the filter cloth with the passage of time. The formation of the filter cake causes the pressure differential, measured across the primary to the secondary sides, to rise. Since an excessive rise in the pressure differential would be undesirable in terms of the inherent limitations upon the facilities that provide the air supply pressure and in terms of a deterioration in filter performance, it is required that the filter cake be removed from the filter cloth. It is also required that the particulate matter be recovered. For these reasons, it is general practice to provide a so-called dust removal mechanism for knocking the filter cake off the filter cloth automatically at such time that the filter cake attains a certain thickness.
Two conventional techniques are available for effecting such dust removal. One is a so-called shaking method wherein the filter cloth is vibrated mechanically to shake off the filter cake. The other is a so-called backwashing method wherein pressurized air is momentarily blown down toward the second side of the filter, that is, from the inner side thereof, to dislodge the accumulated particles from the primary side of the filter by means of the reverse air flow. This latter method is gaining wider popularity owing to its simpler construction.
Before continuing, it should be noted that a filtration apparatus with a somewhat different structure also is available, wherein a number of longitudinally extending bags consisting of filter cloth are provided perpendicular to the walls of the collecting vessel. However, in terms of the overall construction and dust removal technique, this apparatus is essentially no different from that described above.
Generally speaking, in a case where particles have a particle diameter of as large as 10 microns, there is little adhesion among the particles and between the particles and the surface of the filter cloth. For particles of this size, therefore, the filter cake can be dislodged with just slight vibration, irrespective of whether the shaking or backwashing method is employed, and the filtration apparatus may operate without difficulty. For finely divided powders such as pigment power most of whose particle diameters are less than one micron, or carbon black having a particle diameter generally in the millimicron class, the particles exhibit a high physicochemical attraction as well as a high degree of cross-linking which occurs when the particles are irregular in shape, rather than circular. The end result is that the particles have a greater tendency to adhere to one another and to the surface of the cloth filter, making it difficult to dislodge them from the filter. Such particles will be referred to as adherent particles hereafter.
Let us consider the filtration process with respect to such adherent particles. At the early stages of filtration, the filter cake is capable of being dislodged from a comparatively large region of the filter cloth by backwashing. As the filtration and backwashing cycles are repeated, however, the area of the filter cloth from which adherent particles are removed grows rapidly smaller until the filter cake is no longer capable of being dislodged from virtually any part of the filter. This makes a continuously running operation impossible. The cause of this phenomenon is presumed to be that since the wind pressure which acts upon the overall filter cloth area during backwashing is so small, the influence of internal pressure upon the overall filter cloth is almost nil, with the wind employed in the backwashing operation exiting from the primary or outer side of the filter cloth solely from those points offering least resistance. It is from these points alone that the filter cake falls away. As the next filtration cycle proceeds, the flow of contaminated air concentrates at these relatively exposed areas of the filter cloth, from which time onward filtration takes place primarily at these points at a rate much higher than that designed for originally. These occurrences allow the filter cake to harden and cause partial blockage or clogging of the filter cloth pores so that, when the next backwashing cycle takes place, the affected areas of the filter experience almost no air flowing backwardly across the cloth. This, coupled with the hardening of the cake, appears to account for the failure of the dust removal operation. In short, it seems that if dust removal is allowed to take place only in part, then the area of the filter cloth that can be used effectively diminishes until the flow velocity through the filter is no longer true to the proper filtration flow velocity selected when the filtration apparatus was designed. As a result, the filter cake clinging to the filter cloth hardens and grows, gradually diminishing the circulation of air through the filter.
In a case where backwashing is employed to dislodge the filter cake attached to the filter cloth, an outlet pipe through which clean air exits from the filtration apparatus employs flow velocities of from 1 to 3 m/sec for liquids and from 20 to 30 m/sec for gases, which are appropriate for ordinary fluid transport in terms of eccnomy and space limitations. However, when a comparison is made between the outlet pipe flow velocity (say 20 m/sec) and the filtration flow velocity (say 3 cm/sec), it may be understood that the area ratio of the filter cloth to the cross-sectional area of the outlet pipe passage is 2000/3, namely that the filter cloth has 667 times the cross-sectional area of the pipe passage. Moreover, the reversely directed stream of air jetted during the brief backwashing cycle emerges from a backwashing orifice of a diameter smaller than that of the outlet pipe, so that the jetted air stream does not have a flow velocity much different from the flow velocity of the air passing through the outlet pipe. This, coupled with the fact that the filter has a very fine texture, causes the air stream jetted during backwashing to be sealed off so that the air stream can neither flow the length of the cylindrical filter nor pass through the filter cloth from the jetted side. Instead, the air stream acts upon substantially the entire surface area of the filter cloth and develops a flow velocity near that of the abovementioned filtration flow velocity. Consequently, the jetted air stream cannot free the filter cake from the filter cloth, with the result that the pores of the cloth become clogged. A continuously running operation cannot take place unless the filter cloth is cleaned by manually removing the filter cake. Prior efforts at a solution to this problem have resulted in a much larger, complicated and expensive apparatus, for whatever improvement in performance has been achieved.